libstdc++/2694: signedness warning in __advance

libstdc++/2694: signedness warning in __advance

[wolfgang.bangerth]

>Number:         2694
>Category:       libstdc++
>Synopsis:       signedness warning in __advance
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Mon Apr 30 04:56:01 PDT 2001
>Closed-Date:
>Last-Modified:
>Originator:     Wolfgang Bangerth
>Release:        unknown-1.0
>Organization:
>Environment:
gcc 3.0 20010423 prerelease
>Description:
When calling std::__advance(bidirectional_iterator) in some way
or other with an unsigned data type for _Distance, and using -W,
the compiler complains that >=0 is always satisfied. Two
ways to repeat the problem are to compile with -W the
following snippets:
------------------
#include <list>
void g() {
  std::list<int>::iterator i;
  std::advance (i, 1U);
};
-----------------
#include <vector>
#include <map>

void f() {
  std::map<unsigned int, double> m;
  std::vector<std::pair<unsigned int, double> > v;
  v.insert (v.end(), m.begin(), m.end());
};
--------------------

The first gives the messages
tests/deal.II> ~/Config/gcc-ss-linux/bin/c++ -W -c a.cc 
/home/atlas2/wolf/Config/gcc-ss-linux/include/g++-v3/bits/stl_iterator_base.h: In
   function `void std::__advance(_BidirectionalIterator&, _Distance, 
   std::bidirectional_iterator_tag) [with _BidirectionalIterator = 
   std::_List_iterator<int, int&, int*>, _Distance = unsigned int]':
/home/atlas2/wolf/Config/gcc-ss-linux/include/g++-v3/bits/stl_iterator_base.h:356:   instantiated from `void std::advance(_InputIterator&, _Distance) [with _InputIterator = std::_List_iterator<int, int&, int*>, _Distance = unsigned int]'
a.cc:4:   instantiated from here
/home/atlas2/wolf/Config/gcc-ss-linux/include/g++-v3/bits/stl_iterator_base.h:336: warning: comparison
   of unsigned expression >= 0 is always true

The second snippet has an even higher ugliness factor.
>How-To-Repeat:
Compile any of the code snippets above with -W
>Fix:
One would have to specialize the __advance(bidirectional_iterator)
function for signed and unsigned data types. Unfortunately,
the __type_traits class has no indicator telling us whether
a type is signed or not; however, the numeric_limits has
such a field and it could easily be casted to a class.

If someone gives me an indication on how to fix this problem,
I'd be happy to come up with a patch.
>Release-Note:
>Audit-Trail:
>Unformatted: